Process for the hardening of metals



3,3 14,82 7 Patented Apr. 18, .1 967 3 314 827 PROCESS FOR THE HAI KDENING F METALS Hendrik de Vries, Amsterdam, Netherlands, assignor to ShellOil Company, New York, N.Y., a corporation of Delaware No Drawing. Filed June 3, 1964, Ser. No. 372,395 Claims priority, application Netherlands, Dec. 24, 1963,

3 4 Claims. (31. 148-131) This invention relates to a process for the hardening of metals belonging to the Groups 1VB, V-B and VI-B of the expanded form of the Periodic Table of the elements by means of oxygen diffusion. The metals belonging to these groups are: titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten.

The abovementioned metals are very resistant to various aggressive agents such as may occur in the chemical industry. For this reason and also because of other properties, such as a high melting point, these metals are often suitable for use as a material of construction or as a lining for reactors, pipes, valves and other parts for the chemical industry. Naturally since availability and price play an important part in the selection of the metal to be used, in the aforementioned applications of the metals in their pure form, mainly titanium, zirconium, tantalum, and molybdennm have been used so far. In these applicati ns, however, difliculties are often encountered when in addition to corrosion resistance, demands are made on mechanical properties, such as hardness and resistance to wear. For example, the hardness and the resistance to wear of titanium are relatively low and therefore it is not possible for two components which are in sliding contact, such as would occur in valves or in bearings of stirring shafts, to be made of titanium.

Canadian Patent Number 555,952 teaches a method for hardening titanium and zirconium which uses diffusion of oxygen and nitrogen into the metal, as a result of which it becomes possible for oxygen and nitrogen to be taken up interstitially in the metal latice. According to this method, the metal is first heated at 400-800 C. for %16 hours in air or in an oxygenand nitrogen-containing gas. This heating period is followed by heating the metal in argon at 750-1100 C. for it-8 hours. During the first heating period, oxygen and nitrogen are bound at the metal surface; during the second heating period, part of this oxygen and nitrogen diffuses into the metal and is taken up interstitially.

This method, however, has several drawbacks. The amount of gas to be bound during the first heating period cannot be easily controlled, as a result of which undesired surface compounds may be formed. Futhermore, the entire surface which is in the hot zone or heater is hardened. Owing to this, further processing, e.g., machining or bending the metal, is hampered. The hardening of parts already processed and which have to meet rigid requirements as regards dimensions is likewise undesirable in view of the deformations involved in the hardening.

The invention provides the means by which these drawback can be obviated. According to the invention, a film of metal oxide of the metal to be hardened, is applied to the metal surface which is to be hardened. The

metal is subsequently heated at a temperature from 700l C. in an inert atmosphere, e.g.., argon, helium, etc., for a period from 1-60 minutes.

only in that places. Furthermore, the process according to the invention has the additional attraction of requiring only one heating period of short duration.

Preferably the metal oxide before being applied is turned into a brushable suspension. Such a suspension can, for example, be prepared by combining the oxide with alcohol or with water. In this manner, a paintlike substance is obtained, which may, for example, be applied to the surface to be hardened with a brush. Common layer thicknesses are those from 20 to 200 microns.

It should be noted that although any oxide of the metal to be hardened may be used, it is advantageous to use a metal oxide which may convert into a more stable oxide of a lower level of valency. If such an oxide is used, then at a given layer thickness, the quantity of oxygen to be supplied can be determined very exactly, resulting in even better control of the hardening process.

Example I A test specimen of titanium was degreased and provided with a film, about 200 microns thick, of titanium oxide suspended in alcohol. Titanium white (TiO as used in the paint industry was utilized as the oxide. Subsequently, the test specimen was heated at about 900 C. in an atmosphere of argon (99.9%) for 6 minutes. After cooling down, the Vickers hardness (200 g.) was 350 kg./mm. at 30 microns below the surface and 300 kg/mm? at microns below the surface. The hardness before the hardening process was 130 kg./mm.

Example II A test specimen of tantalum was degreased and provided with a film, about 200 microns thick, of tantalum oxide suspended in alcohol. Subsequently, the test speciment was heated at about 900 C. in an atmosphere of argon (99.9%) for six minutes.

After cooling down, hardening relative to the starting material was found to have occurred.

Example III A test specimen of zirconium was degreased and provided with a film, about 200 microns thick, of zirconium oxide suspended in alcohol. Subsequently, the test specimen was heated at about 900 C. in an atmosphere of argon (99.9% for six minutes.

After cooling down hardening relative to the starting material was found to have occurred.

I claim as my invention:

1. A process for hardening metal surface of metals selected from the group consisting of titanium, zirconium, vanadium, tantalum, chromium, molybdenum and tungsten, by means of oxygen ditfusion, comprising: coating said metal surfaces with a brushable suspension of metal oxide corresponding to the metal surface to be hardened; and, subsequently heating the metal oxide coated metal in an inert atmopshere at a temperature from 700-1100 C. for a period from 160 minutes.

2. A process according to claim 1 wherein said metal is titanium, the brushable suspension is titanium oxide suspended in alcohol and the heating ternlperaure is around 900 C. and the heating time is around 6 minutes.

3. A process of claim 1, wherein said metal is zirconium, the brushable suspension is zirconium oxide suspended in alcohol and the heating temperature is around 900 C. and the heating time is around 6 minutes.

4. A process of claim 1, wherem said metal is tantalum, the brushable suspension is tantalum oxide suspended in alcohol and the heating temperature is around 900 C. and the heating time is around 6 minutes.

References Cited by the Examiner UNITED STATES PATENTS 2,163,266 6/1939 Bartels 148-6 2,681,876 6/1954 De Santis et a1. 148-133 3,111,434 11/1963 Takao et a1. 14813.1

DAVID L. RECK, Primary Examiner. R. O. DEAN, Assistant Examiner. 

1. A PROCESS FOR HARDENING METAL SURFACE OF METALS SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM, VANADIUM, TANTALUM, CHROMIUM, MOLBDENUM AND TUNGSTEN, BY MEANS OF OXYGEN DIFFUSION, COMPRISING: COATING SAID METAL SURFACES WITH A BRUSHABLE SUSPENSION OF METAL OXIDE CORRESPONDING TO THE METAL SURFACE TO BE HARDENED; AND, SUBSEQUENTLY HEATING THE METAL OXIDE COATED METAL IN AN INERT ATMOSPHERE AT A TEMPERATURE FROM 700-1100*C. FOR A PERIOD OF FROM 1-60 MINUTES. 